Many buildings, particularly relatively small buildings such as single-family houses, have a single heating, ventilation, and air conditioning (HVAC) unit that is controlled by a single thermostat. The HVAC unit typically comprises some type of fluid temperature conditioning device, such as a furnace for heating air, a boiler for heating a liquid or steam, or an air conditioner having an evaporating coil for cooling air. If the fluid is air, it is typically ducted to various locations within the building, or if it is liquid or steam, it is typically piped to heat exchangers at various locations in the building. The thermostat in this type of space conditioning system is typically positioned at a location where the heating and cooling loads are representative of the entire structure. For example, the thermostat may be installed in an interior room away from windows and doors that would tend to influence the sensed temperature. The HVAC equipment then controls the heating and cooling of the entire structure according to the thermostat signal received from the single location.
However, a single thermostat location may not accurately represent the heating or cooling needs throughout the structure. Other locations of the building may have significantly greater or lower heating and cooling loads than exist at the location of the thermostat. For example, rooms having a larger surface area of windows, or rooms having a greater area of exterior walls, may require greater heat inputs to maintain the desired temperature. Similarly, rooms facing south or west, or rooms that are on an upper story, may require greater cooling inputs to maintain the desired temperature. In cases where the HVAC equipment is controlled only by a single thermostat, the heating or cooling supplied to each individual area of the building will be based on the heating or cooling needs at the thermostat location and not on the actual heating and cooling needs of each individual area. As a consequence, the heating and cooling loads of individual areas of the structure may not be satisfied and the temperature of these areas will tend to deviate from the desired temperature.
In some situations, it may be desired to control different locations within a building at different temperatures. For example, rooms that are seldom occupied may not need to be maintained at the same temperature as rooms that are frequently occupied. Energy that is used to heat or cool these unoccupied rooms is not used effectively or economically. Also, rooms may be occupied by people having special temperature needs, such as an elderly person or an infant, that are preferably maintained at a different temperature than the rest of the building. However, a system that has only a single thermostat is generally unable to accurately control different locations in the building at different temperatures.
One solution to this problem is to utilize HVAC zone control. Rather than having a single thermostat controlling the HVAC equipment, multiple thermostats are positioned at locations within the building that are expected to have different heating and cooling loads. Although it is possible that each of these thermostats could control a separate fluid temperature conditioning device such as a separate furnace or air conditioner for each zone, that approach is generally neither efficient nor economical. Rather, most commonly the ductwork or piping that is used to transmit the conditioned fluid to the building spaces is configured with controls to adjust fluid flow to the various zones of the building corresponding to the various thermostats. For example, air ducts may be configured with controllable dampers that are capable of opening and closing to control the flow of air to a particular zone within the building when the thermostat in that zone calls for conditioning.
A system having HVAC zone control generally requires the use of a zone controller to receive the signals from the various thermostats, control the operation of the heating or cooling device, and control the distribution of the conditioned fluid through the ductwork. The zone controller typically comprises electronic circuitry for evaluating the heating or cooling needs of the various zones of the building and for determining an appropriate control of the heating or cooling device and the dampers or valves that control distribution. The distribution control where the conditioned fluid is air is typically accomplished with a duct damper. A duct damper typically comprises a variable obstruction within the duct that can be actuated to one position where there is relatively little resistance to air flow within the duct, and can be actuated to another position where there is relatively great, or complete, resistance to air flow. Duct dampers can be controlled by any of a number of actuation means, including electronic, pneumatic, or mechanical. The HVAC zone controller generally is configured to open or close a duct damper in order to effectuate control over a zone in response to thermostat signals.
In addition, some HVAC systems are equipped with a multiple stage fluid temperature conditioning device that has multiple heating or cooling output stages. For example, a furnace may be provided with multiple heat output stages such as where a variable flow gas valve or multiple burners are selectively controlled to provide a relatively lower heat output stage and a relatively higher heat output stage. Similarly, a multiple stage heat pump may be provided that has multiple compressor speeds where the different compressor speeds are controllable to vary the output rate of the device. In some other circumstances, multiple conditioning units are provided where the operation of a single device constitutes a relatively lower stage of output and operating multiple devices simultaneously constitutes a relatively higher stage of output. Oftentimes, heat pump systems are provided with secondary electrical resistance heating that can be engaged to provide higher stage heat output when necessary, particularly when the outdoor air temperature is low and the heat pump efficiency is low. Other types of multiple stage HVAC equipment exist.
In an HVAC system having both zone control and a multiple stage fluid temperature conditioning device, it can be challenging to determine the proper control strategy for the multiple stage fluid temperature conditioning device. There is a need for improved controls for multiple stage conditioning devices used in HVAC systems having zone control.